it's is helpful as it prevents some company from forking the code, closing it and then “defeating” your project using aggressive marketing or dirty business practices (wine had companies that forked the code and not give back which eventually led to it being relicensed under the LGPL).
It will also make it easier for the community to trust and invest (code or money) in the project because there is less of chance of him going closed source like reddit.
Linus torvalds (creator of linux) called the GPL “one of the defining factors in the success of Linux” he also talked about how it helps motivate contributors:
A developer would feel that the big company is going to take advantage of their work, said Torvalds. "The GPL ensures that nobody is ever going to take advantage of your code. It will remain free and nobody can take that away from you. I think that's a big deal for community management.
I think what makes copleft work well is that it provides a strong incentive to contribute back (based on casual observation , there appears to be a correlation between a large project being copyleft and it being high quality), it is analogical to a "tit for tat" strategy which is reportedly the most effective for at least some games in game theory.
Also i think a dislike for inequality (even just a perceived one) seems deeply ingrained in us, that's true for even monkeys.
However there are some cases where a permissive license might be better (even the FSF agrees).
Further reading:
A typical discussion of licensing, ending with the project converting to a copyleft license.
Open source licensing and why we're changing Plausible to the AGPL license
Changing to AGPLv3 and introducing the Enterprise Edition
The Land Before Linux: Let's talk about the Unix desktops - On why the permissive BSD license arguably caused the failure of BSD based operation systems and the success of Linux